1. Technical Field
This invention relates to floating platforms that serve as landings for vessels, and more particularly to a moored floating platform having a configuration of pontoons and bridge assemblies that reduces platform motions caused by waves and wave phenomena, thereby allowing a more nearly level platform to and from the vessel for passengers and stevedores.
2. Background Art
Floats used for vessel landings are constructed either as walkways, supported at intervals by buoyant cells, or barge-type hulls where the deck of the barge is the landing area. Landings constructed as walkways are typically simple surfaces supported at regular intervals by floatation cells that may be Styrofoam or sealed chambers. The walkways are segmented and connected at joints. Each walkway segment is fitted with one or more buoyant cell. The walkway lands on top of the buoyant cell(s). The landing, acting over the length of the walkway with the buoyant supports, does not act as a structural monolith, where moment and shear loads at one end are transferred to the opposite end. In this sense these landings may be categorized as articulated, such that their components are connected by a series of shear loaded joints which do not transfer moment.
Barge-type hulls act as monolithic structures where moment and shear are transferred through the structure, and loads applied at one extreme end influence the reaction at the other extreme end. These are single structures with internal subdivisions to form tanks and internal boundaries. Variations on this design may include a series of individual pontoons linked together to form a single but composite structure. This type of composite structure links the individual pontoons closely together so that they tie together to form a single barge-like structure. The characteristic of these landing floats is that a continuous buoyant volume is developed along the length of the float body. These floats have the appearance of a barge where the immersed dimensional envelope defines the buoyant volume.
These floating bodies, when acted upon by waves, rock, roll and oscillate on the water surface, generally with six degrees of freedom, namely roll, pitch, yaw, heave, surge and sway. Waves force these bodies to experience motions on the basis of periodicity and wave length. And yet each floating body also has its own natural oscillation period. When the forced wave period and the natural period of the floating body coincide, wave induced loads on the floating body reach maximum amplitudes.
In wave mechanics, the period and wave length are directly related. Short wave lengths have short periods and high frequencies. The effect that a wave form has upon a floating body can also be expressed in terms of the wave length, since period and length are related. Waves that are extremely short, compared to the length of the floating body, will have little effect upon the body. This is due to the fact that the body is being acted upon by a number of waves simultaneously along its length and their net effect is to cancel each other out. The longer the wave length the less the phase differences between competing waves along the length of the body. Long wave lengths, compared to the length of the body, will have a greater effect upon motions.
The wave form exerts a force upon the floating body that is both buoyant and inertial. The buoyant force is created because the wave form elevates or depresses the water profile around the floating body thereby altering the buoyant loads along the body""s length. A floating body that encounters a wave crest at the front of the body will experience an upwards load that tends to lift the front relative to the rear. The wave creates a xe2x80x9ctrimming momentxe2x80x9d that tends to trim the front up and the rear down. As the wave creates passes under the body, that trimming moment will eventually cause the rear to trim up and the front down.
In this way the wave form can be seen to create a rocking motion, or pitch, along the length the body. As the wave form passes along the length of the body, it also alters the net elevation of the water level so that the wave tends to lift the body. In this way the wave form can be seen to create an up and down motion, or heave, on the body.
The inertial force is created by the water particles themselves as they move in an orbital path, pushing against the body""s surfaces. When the wave front is acting against the front of the floating body, the wave particles are also pushing against the front as they move in their orbit. The push action of the water particles tends to force the front of the body backwards. As the wave passes under the body, the push action of the water particles in the trough of the wave tend to push the body forward. In this way the wave form can be seen to cause a fore and aft motion, or surge, in the floating body. If the wave front acts slightly askew to the alignment of the floating body, the wave particles tend to force the front to one side and then back to the other side as the wave passes under the body. In this way the wave form can be seen to cause a twisting motion, or yaw, in the floating body.
The wave front that strikes the floating body on the side exerts an effect similar to a head on encounter. The wave front buoyant force lifts one side of the body then the other side as the front moves under the body. This creates a rolling motion in the body. The side inertial force of the wave particles tends to push the body sideways, thus creating a swaying motion. As the wave front moves under the body, it also alters the net elevation of the water level so that the wave tends to lift the body, thus creating heave. Where the side wave hits slightly askew, the body tends to yaw.
All floating bodies have natural periods whereby they oscillate in uniform harmonic motion to all six degrees of freedom. Waves force a floating body to experience these motions and the body tends to oscillate at its natural frequency. Wave frequencies that are out of phase with the body""s natural period tend to have less effect upon the body""s motion than those frequencies that are closer to the body""s natural response frequencies.
Motion dampening of a floating body is traditionally of three types. The first is to design a body""s natural period to be significantly displaced from the peak period of the design wave form. The second is to develop systems internal to the floating body that respond with counter moments to the buoyant forces that the wave front exerts upon the body. The third is to utilize keels to resist rolling motions.
Each approach has limitations when applied to moored floating platforms. These limitations are a consequence of economic and mass limitations. In simple terms, floating moored vessel landings need to be compact, inexpensive and resilient to vessel impacts. As a consequence, moored vessel landings are traditionally designed as simple cubic structures, described as barges tied off to piles.
The natural period of rolling or pitching of a floating body is dependent upon the distribution of its mass. Basically, when the body rolls or pitches, it describes an arc of rotation about a center, generally located near the center of gravity. The period of the rotation is assumed to be harmonic. According to the laws of simple harmonic motion, or SHM, the period of oscillation is a function of how the mass is distributed about the focal point of the rotation. In a compound body in rotation, the distribution of mass can be assumed to be located at a point from the mass center called the xe2x80x9cradius of gyration,xe2x80x9d or gyradius. The gyradius is the point located from the motion center where the entire mass of the body appears to be located. The position of the gyradius is solved by dividing the mass moment of inertia of the body by its mass and taking the square root of the quotient.
Increasing the mass moment of inertia, and consequently the SHM period, involves increasing the dimensional and cubic measurements of the floating body.
The second method of controlling or dampening body motions is developed on the principal of creating counter moments caused by the transfer of a mass of fluids that opposes the externally created displacement force, such as wave induced motions. These internal forces may be characterized as fluid masses sloshing from side to side in opposition to the frequency and period of wave generated forces. Simply stated the internally confined fluids have a transfer period, from side to side of the floating body, that opposes the externally generated force of the natural wave period. Thus, when the wave period forces one side up, the internally generated slosh of fluids forces that same side down.
A third method of controlling or dampening body motions is to use the resistance of keels to create force in opposition to the rolling motion of the body. In typical barge design. keels are not used, either as centerline keels or bilge keels. This is primarily because of restriction to access of the barge side and the exposed nature of such keel like structures.
Heretofore, conventional designs utilize mechanisms and designs that increase the mass and complexity of the basic hull barge form in order to reduce the effect of externally applied wave forces.
Float designs where the wave front energy is used to alter the periodicity of response and where wave front buoyant loads are altered to change the net trimming or rolling moments is not evident in conventional moored float designs.
It is shown in this invention that a complex solution of motion dampening is possible in the form of an arrangement of buoyant cells connected by bridge assemblies. The inventive solution embodies a disruption of wave induced buoyant loads, period interference. and turbulence drag.
The primary object of this invention is to reduce significantly the response characteristics of a floating platform, making it more stable with less wave induced motions. It is a further object to achieve this first object while also reducing construction and material costs over conventional designs.
Another object of the invention is to teach the design of a system of floating cells that alter the natural response period of the composite floating body.
A further object is to provide a type of floating body where the arrangement of floatation cells interferes with the natural wave period, reflecting portions of the wave front against the original front.
Yet another object is to describe a system of bridge works that provide a rigid system of connected floatation cells that are acted upon by wave forces. The bridges separate the floatation cells so that the frequency of encounter of the wave front to each cell is at odds with the time period with which the wave traverses the bridged gap between cells.
Another object of the invention is to teach the advantage of open spans between buoyant cells where keels may be used as broad, flat and vertical members that also connect the respective cells. These broad flat members set up turbulent eddies and restrict flow by offering a flat resistive keel to lateral, or xe2x80x9crolling,xe2x80x9d motions.
Still another object of the present invention is to teach the use of broad, flat and vertical members to connect the respective floatation cells, as reflective boundaries to wave fronts approaching from the side. These flat boundaries reflect some percentage of the wave front back upon the advancing front.
Another object of the invention is to teach design procedures that make it possible to install floatation platforms with a minimum of material needed to reduce wave induced motions.
According to the present invention, a lightweight pontoon assembly can be formed by a sequence or series of floatation cells, every adjoining two cells separated by rigid bridge spans so that the wave induced forces that act upon the pontoon assembly are altered. This alteration of applied force occurs according to the sequence, or spacing, of floatation cells and also alters the periodic response characteristics of the pontoon assembly to the wave energy. The arrangement of floatation cells also creates reflected wave fronts that act against the oncoming wave and reduce the overall wave effect.
In addition, the rigid girders forming the bridges that connect the floatation cells are vertically deep members. These girders create turbulence and eddies as they move against the wave front. The eddies and turbulence create resistance to the wave induced motion as well as opposing wave making, all of which dampens motion.
The pontoon assembly is acted upon by wave fronts that act perpendicular or parallel to the major pontoon axis. A wave front with a direction of motion parallel to the pontoon major axis, or length, causes the pontoon to pitch, surge and heave. A wave front with a direction of motion perpendicular to the pontoon major axis causes the pontoon to roll, pitch, heave, sway and yaw.
In understanding pitching motion, floatation cells should be regarded as independent bodies, each acted upon by a wave front independent of the other cells. The cells may be spaced at such a distance that for a given wave length, hence frequency, the cells experience non-contributory motions. Contributory motions may be defined as those that combine to amplify any of the six freedoms. For example when a given first cell is moving up, a given second cell is moving down. If a line is drawn between the first and second cells, it is seen that the line pitches down. In this case the spacing of the cells enhances a rotational pitching motion relative to the two cells. If the spacing of the cells is such that both exhibit the same vertical motion, then a line drawn between the two cells would show no pitch down or up, so that the cell spacing dampens the pitching motion relative to the two cells.
When the cells are not seen as independent bodies, but rigidly connected, the movement of each influences the movement of the other. Rather than an imaginary line drawn between the cells, it is now proposed that a rigid connection exist between cells. When two cells so connected are acted upon by a wave front and are spaced so that they experience the same vertical motion, the pitching motion of the assembly can be, dampened.
In any arrangement of one or more rigidly connected floatation bodies, the wave profile acting over the length, or major axis, of the arrangement causes its center of buoyancy to oscillate about the midpoint region of the assembly. While the center of gravity of the pontoon assembly remains fixed, the oscillation of its center of buoyancy causes a couple to exist between gravity (downward force) and buoyancy (upward force.) This couple provides the force that generates a pitching motion in the pontoon assembly along the major axis.
For any given wave length, or frequency, the spacing of floatation cells in the pontoon assembly can be set so that the movement or oscillation of the center of buoyancy about mid point is minimized. This in turn minimizes the pitching couple and the resultant motions. The spacing of floatation cells can be expressed as the product of buoyant length and spacing to the square of wave length.
The arrangement of the buoyant cells along the major axis also affects the mass moment of inertia of the pontoon assembly on the major and minor axis. These moments of inertia determine the natural response frequencies, pitching and rolling, in terms of simple harmonic motion.
For a given wave length, a number of floatation cells can be sequentially and rigidly connected at distance combinations where rotational and linear motions are non-contributory between the individual cells. This would cause a uniform dampening of wave induced motions.
The fundamental concept in dampening pitching motion is that the trimming and buoyant forces, summed between the rigidly connected buoyant elements, are minimized due to spacing of buoyant cells distributed along the length of a given wave.
In understanding rolling motion, consider that the non-buoyant structure of the bridge girders is not acted upon by wave forces which cause rolling, pitching and other buoyancy related phenomenon. So that the bridge structure is being forced through the water because it is rigidly attached to buoyant elements. If the bridge structure presents a broad and flat surface which opposes this motion due to resistance, turbulence, eddy making and wave making, similar to a deep keel, then it can effectively dampen the motion it opposes.
In addition each buoyant cell and each bridging girder presents a broad flat surface to wave motion. Because of the dampening characteristics of the present invention these flat surfaces are not moving at the same orbital velocity of the wave particles. This causes some of the wave energy to be reflected off the flat surface and interferes with the primary wave train. This interference pattern can increase the dampening of the wave induced motion by suppressing the energy of the incident wave front.
The second concept is that the natural period of the spaced buoyant cells is significantly displaced from the frequency of the given wave. An extremely narrow body acted upon by a given wave front will experience optimally linear displacements because the small mass moment of inertia and gyradius will displace the natural motion period of rolling or pitching far from the frequency of the given wave. Where the buoyant profile of a body is interrupted, the periodic action of wave force is interrupted, altering the simple harmonic motion response of the body. This may be characterized as an assembly of pendulums acting in series where they impact at out-of-phase intervals.
These float assemblies are designed primarily for use as landings and terminals adjacent to shorelines and beaches. The presence of a shoreline causes a wave front refraction so that the direction of wave motion is bent towards the shore and always tends to be perpendicular to the shoreline. This means that the line of waves created is almost always parallel to the line of the shore. Vessel moorings and landings can be established as piers, where they extend their major axis perpendicular to the shoreline, or as wharves where they extend their major axis parallel to the shoreline. The alignment of the landing determines the principal motion, pitch or roll.
The arrangement of the pontoon assemblies of this invention dampens motions on both the major and minor axis because of the discontinuous arrangement of buoyant cells and their completely rigid connections.
Each category of motion, and motion dampening, is frequency related. A non-dimensional parameter is created which relates dampening functions to the wave frequency. This relational parameter is the ratio of overall pontoon assembly length to wave length, for pitching motions, and overall pontoon assembly width to wave length, for rolling motions.
All wave dampening characteristics are compared to a conventional float where its dimensional envelope is entirely buoyant, and barge like.